Hi.
I use my car ALMOST solely for work, which means motorway driving, as a work all over the place. My car is the 1.6 petrol edition and I get 41MPG on a good day 40 is more or less the average. Needless to say that would drop thru the floor if I used it to tuttle round town.
Based upon this I don't think you getting the best value from a diesel engine. I would be attempting to get significantly great mileage if I had a diesel.
To achieve 40 MPG with my 1.6 petrol Edition I have to restrain myself when driving, which means, I find a nice big truck chugging along about sixty (which I believe is more realistic for optimal consumption) and set my internal cruise control select my favourite radio station and settle in for a stress free ride.